"[Application] is not responding" dialog displayed under the application's dialog
In some cases, the "... is not responding" dialog box shows up under the unresponsive application's dialog box.
It's trivial to reproduce it with LibreOffice:
- Start LibreOffice
- Open some modal dialog box - e.g., file open
- Freeze LibreOffice, e.g.
killall -STOP soffice.bin
You'll see the main window become darker, but you probably won't see the "not responding" dialog box.
Screenshot: